1. Field of the Invention
This invention relates generally to the field of fluid flow measurement and more particularly to a novel acoustic method and instrument for measuring the mass flow rate and certain other related variables of a fluid flowing through a conduit.
2. Prior Art
There are numerous methods of measuring fluid flow. These measurements are generally inferred from the pressure drop measurement across an obstruction placed in a flow path. Examples are venturi meters, orifice meters, nozzle meters, pitot tube meters, or devices which use suction effects to measure a pressure differential. These "head" type meters all have one common disadvantage, that being flow obstruction. Accordingly, the flow measurement obtained is not average flow, but modified flow, thus requiring a correction factor to obtain a relative indication of flow. Another distinct disadvantage of these devices is that they lose accuracy as the diameter of the flow line and velocity of flow increase.
Mass flow is usually measured by positive displacement meters. In these meters, a movable area of known dimension is mounted in the flow line in such a way that the action of the fluid on the area produces a torque or other positive form of measurable pressure. Included in this class of meters are turbine, piston, and cam meters. The common disadvantage of such positive displacement meters is, again, flow obstruction and loss of accuracy as the conduit size increases. Also, none of the devices measure average flow, but rather flow in a confined area which must then be correlated to an average flow.
Ultrasonic flow measurement techniques have also been devised. An article entitled "Electronic Flowmeter System", in the Review of Scientific Instruments, March 1954, Vol. 25, page 201, for example describes an ultrasonic flow measurement system involving the technique of electronically switching from one transducer to another attached to the outside of a conduit. This system measures the phase relationship between upstream and downstream signal transmissions which can be correlated to obtain a measurement of the fluid velocity. If a sufficiently high carrier frequency is employed, this system has the ability to measure flow accurately in conduits as small as one-quarter inch in diameter. The sensitivity of the receiving crystal is a limiting factor, however, which makes the shielding requirements of the electronic circuitry too severe for wide spread use of the system.
Another article entitled "Ultrasonic Flowmeter", in Instruments and Automations, Vol. 28, page 1912, November 1955, describes an ultrasonic flowmeter system which employs two transmitters and two receivers attached to the inside of the conduit. This system requires pipe penetration and measures phase relationship of the transmitted and received ultrasonic signals.
A system which utilizes the flow measuring techniques outlined above in conjunction with a density measuring system including a transducer mounted on the conduit wall to obtain mass flow rate is described in "Process Instruments and Control Handbook", at page 4-89, McGraw-Hill Publishers, 1957. By taking the output of the two systems, mass flow rate is computed electronically. This device requires pipe penetration which is a disadvantage.
Several other ultrasonic flow measurement systems which use the principle of dual path or single path transmission of ultrasonic signals within the conduit have been described in the literature as follows:
Forgacs, R. L., "Precision Ultrasonic Velocity Measurements", Electronics, page 98, Nov. 18, 1960 PA1 Messias, H. F., "Ultrasonic Measure Flow Velocity of River", Electronics, page 56, Oct. 13, 1961 PA1 Brown, A. E., "Dual Path Ultrasonic Flow Measurements of Fluid Flow", Review of Scientific Instruments, page 1181, September, 1966 PA1 Brown, A. E., "Ultrasonic Flow Measurement", Instrument and Control Systems, page 130, March 1967.
Ultrasonic flow measurement systems are also found in the prior patent art. Flow measurement systems of this class, for example, are described in the following patents:
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